1. Field of the Invention
The present invention relates to power supplies, and more particularly, to power supplies for integrated circuits.
2. Description of the Prior Art
A variety of integrated circuit applications often require power to be supplied at two or more different supply voltages. For example, multiplexed liquid crystal displays (LCD's) for digital quartz integrated circuit watches often require two supply voltages, typically -1.55 volts and -3.10 volts. The smaller voltage can be supplied directly by a battery such as a 1.55 volt silver oxide battery. The larger supply voltage is typically generated by a switched capacitor voltage doubler which doubles the voltage supplied by the battery.
These voltage doublers often require two relatively large capacitors, typically 0.047 microfarads in size. Because of their relatively large size, these capacitors are typically fabricated as ceramic chip capacitors which generally are not a part of the integrated circuit chip. Thus, capacitors which are external to the chip must be made separately and physically connected to the integrated circuit chip. This can increase the cost and complexity of the manufacturing process and increase the cost of the final product. Furthermore, the external connections can break or come apart, thereby decreasing the reliability of the product.
In addition, the external capacitors often are used to drive the displays directly. As a display draws current from the capacitors, the capacitors need to be refreshed. As a result, the switching circuitry which refreshes the capacitors consume current which can shorten the life of the battery.